This invention relates generally to the field of variable resistors. More particularly, the invention relates to a ratcheting mechanism for variable resistors of the type actuated by a worm screw engaging a spur gear.
Variable resistors or potentiometers of the worm screw-actuated type are well-known in the art, as exemplified in U.S. Pat. Nos. 3,179,910 - Grunwald; 4,004,264 - Hogue, et al; and 4,114,132 - DeRouen et al (the last named patent being commonly assigned with this application). Typically, in such potentiometers, the worm screw actuator must be rotated through several complete revolutions to cause the rotor-mounted contact or wiper to travel the length of the resistive element, the latter traversing a generally arcuate path of less than 360 degrees. To prevent discontinuities in resistance, end stops or other means are provided to prevent the wiper from leaving either end of the resistive element.
It has been found that a clutch or ratchet mechanism is necessary to prevent damage to the gears from continued rotation of the worm gear after an end stop has been reached.
A variety of types of such clutch or ratcheting mechanisms has been devised, some of which are exemplified in the aforementioned prior art patents. The approaches taken by the prior art have sought to provide a ratcheting mechanism capable of high reliability, in the sense of allowing gear slippage only at the end stops and positive gear re-engagement when the direction of screw rotation is reversed. A further objective is to provide such a function with a mechanism of compact size, as the further miniaturization of such potentiometers progresses. Related to both of these objectives is the goal of simplifying the structure as much as possible.
Another consideration in the design of such devices is the need to provide some audible indication that the device is ratcheting. Since these miniature potentiometers are sealed units, the mechanism is not visible to the operator, who must, therefore, have some other indication that an end stop has been reached. Thus, another design goal has been to provide an audible "click" when the ratcheting mechanism is in operation.
The prior art devices have satisfied one or more of the aforementioned goals with varying degrees of success. For example, the device disclosed and claimed in the above-mentioned patent to DeRouen et al has achieved reliability with a structure that is both relatively simple and compact, and which also yields a good audible "click" in the ratchet mode.
However, the DeRouen et al. device uses an over-center spring both for the wiper contact and for engaging the ratchet mechanism with the worm screw at the limits of rotor travel. While this arrangement is entirely acceptable in many applications, in other applications it is more desirable to use an off-center spring. While reasons for such desirability vary, spring shape, size, and configuration are often constrained by the spring's primary purpose as an electrical contact, with its ratchet-engagement function being subservient to this purpose. Thus, in many instances, an off-center spring is the optimal design from the standpoint of electrical function, while in other instances, packaging constraints may dictate the use of an off-center spring.
Thus, it would be of benefit to the industry to provide a potentiometer ratcheting mechanism which is both reliable and adapted for miniaturization, while also having the virtues of simplicity of structure and economy of manufacture. In addition, it would be advantageous to provide such a mechanism which is adapted for utilizing an off-center spring in the dual role of electrical contact element and ratchet mechanism engagement member.